Chemical vapor deposition (CVD) refers to processes in which layers are formed on the surfaces of substrates by chemical conversion of a precursor chemistry that contains the elements of which the desired layers are comprised. A common example is the formation of silicon dioxide (SiO2) from chemical reactions between the precursor hexamethyldisiloxane, or HMDSO (O[Si(CH3)3]2), and oxygen (O2). In this case, the silicon and oxygen combine to form the solid SiO2. The remaining methyl groups can also react with the oxygen to form various waste gasses (CO, CO2, HO, etc.) that are then removed from the reactor.
To improve the efficiency of the reaction and the quality of the deposited layers, the reaction can take place in an atmosphere of ionized gas (plasma). A typical gas for generating the plasma is Argon. Reactive gasses that may be used in the chemical reactions can also be added. Suitable plasma will have a high density of energetic electrons. The electrons undergo collisions with the precursor, and other gasses containing the elements of interest, thus ionizing the gasses and dissociating their molecules into lighter fractions. This produces radicals of the elements of interest, which form the desired compounds more readily than they would without being subjected to the plasma. Adjustments of the ratios of gasses, pressure, or power parameters can be made to affect the rate of deposition and film properties.
Plasma is generated within a vacuum chamber, at a pressure in the range of about 1-100 mTorr, by providing ionizable gas and electrodes within the chamber and then applying an electrical potential between the electrodes. When the electrical potential is applied, electrons are emitted from the surface of the cathodic electrode and will be accelerated by the electric field between the cathode and anode. When electrons of sufficient energy collide with process gas, the gas can be ionized and additional free electrons are generated. The new electrons are also subjected to the accelerating electric field and can act to ionize additional gas molecules. Under suitable conditions, this process quickly cascades to form dense plasma. It is common to further provide magnetic fields that are configured to alter the trajectories of the electrons so as to entrain them within a desired work area and increase the probability of undergoing ionizing collisions, hence, improving the efficiency of the process.
A variety of configurations can be found in the prior art for generating plasma. But, they generally have shortcomings that limit their performance for PECVD processes. A summary of such sources can be found in U.S. Pat. No. 7,327,089 (also referred to here as the “'089 Patent”), hereby included in its entirety by reference.
In the '089 Patent (and subsequent continuation U.S. Pat. No. 7,411,652 (also referred to here as the “652 Patent”)), a source comprising an electrode confined within a cavity is described. The cavity is open to the process area of the vacuum chamber through a nozzle that restricts the flow of plasma. The nozzle also limits the flow of sputtered materials out of the cavity and the flow of the chemical precursor into the cavity. A means for supplying the process gasses within the cavity is provided. The precursor is supplied to the chamber outside the cavity. This configuration addresses the concerns of reacted materials forming on the electrode and of sputtered material contaminating the film that forms on the substrate. This source also incorporates a magnetic field that provides magnetic confinement of the electrons within the source to enhance the efficiency in generating dense plasma. The magnetic field configuration also provides guidance for a portion of the plasma to flow out of the cavity towards the substrate.
An issue with the source described in the '089 Patent is that it relies on an anode that is outside the cavity of the source. Being outside the source, the anode is subject to deposition of dielectric coatings created by the process. As one skilled in the art can appreciate, dielectric coating on anodes invariably leads to process instability and unacceptable non-uniformity. Additionally, the electrode disposed within the cavity is subject to oxidation (or other reactions) from the processes gasses being used. Both problems are alleviated by connecting two of the sources together with an alternating-current (AC) or bi-polar pulsed power supply as described in the '652 Patent. In this configuration, the electrode in one source is at cathode potential while the electrode in the other source acts as the anode. The two electrodes switch polarity on each half-cycle of the power supply. On the cathodic half-cycle, the electrode experiences sputtering. This can at least partially clean off undesired reactants.
An important feature on the two-source configuration described in the '652 Patent is that the two sources can be magnetically linked by means of having opposite magnetic polarities. This is an efficient way of directing the working electrons out of the sources and into the work area, since the electrons will follow the magnetic field as they move between cathode and anode. Although the two-source configuration reduces the coated or reacted electrode problems, it creates another problem. The uniformity of the electron travel between sources is highly dependent on the uniformity of the magnetic field linking the sources. Even if great care is taken to make the magnetic structure within each source highly uniform, it can be extremely challenging to make the linking magnetic field between the sources uniform. This is due to the distance between the sources. Since the strength of the magnetic field (flux density) diminishes as a function of the square of the distance, small variations in the distance between sources can result in significant changes in magnetic flux density. This results in unacceptable changes in the process. The two sources must therefore be precisely parallel, which implies that they must also be precisely straight in order to maintain a uniform process. The longer the sources get, the more challenging it gets to achieve and maintain adequate precision.
In German Patent DE 199 28 053 (also referred to here as the “'053 Patent”), a plasma source comprising cathodes and redundant anodes arranged within a cavity is disclosed. This source comprised three parallel cathode electrodes constructed of magnetically permeable material, such as steel. One of the three is a solid slab that can be disc shaped. Alternately, it can have a rectangular or oval shape. For simplicity, only the disc shaped configuration will be discussed here. The other two cathodes are annular shaped and have substantially the same thickness and outer diameter as the disc-shaped cathode. Disposed between and connecting the cathodes are permanent magnets arranged in a ring that interface the cathodes on the flat surfaces and along the outer diameter of the cathodes. The assembly of the cathodes and magnets produce an enclosed cylindrical structure with a cavity in the center, which is open at one at one end of the cylinder. This assembly is simultaneously the main electrode and magnetic circuit of the plasma source. Disposed within the cavity and between the cathodes are additional electrodes that are mutually electrically isolated from each other and from the cathodes. The electrodes are cylindrical shaped loops with an inner diameter slightly larger than the inner diameter of the cathodes and outer diameter less than the inner diameter of the ring of magnets. Their lateral dimension is less that the distance between cathodes. These electrodes, which serve as anodes, are disposed between and concentric with the cathodes. The electrode anodes are electrical components only and are not part of the magnetic circuit.
In the disc shaped configuration disclosed in the '053 Patent, the magnetic polarity is arranged so that the field permeates the permeable cathodes in a radial direction and emanate out of the cathodes at or near the inner diameter surfaces. The field lines form arcs in the space between the cathodes, substantially parallel to the axis of symmetry of the assembly and substantially bridging the inner diameter surfaces of the anodes. In this way, electrons emitted from the cathodes are entrained between the cathodes since the electrons cannot readily cross the magnetic field lines to reach the anode. The result is a build-up of electron-rich plasma within the cavity. A portion of the electrons escapes the source along its center axis and towards the substrate.
In the disc shaped configuration disclosed in the '053 Patent, a DC power supply is connected between the cathodes and ground and maintains the cathodes at cathode potential. Two separate square-wave power supplies are connected between the anode electrodes and the cathodes. These power supplies are alternately pulsed so that one electrode is at anode potential while the other is at cathode potential. Periodically driving one anode at a time to cathode potential allows the system to always maintain an active anode while sputter-cleaning the other. In this fashion, the anodes are kept at least partially clean and functional.
Since all electrodes are in close physical proximity in the disc shaped configuration disclosed in the '053 Patent, establishing and maintaining adequately uniform magnetic pathways for the electrons becomes much easier. The '053 Patent appears to have remedied the issue of non-uniformity of the '652 Patent, while maintaining the benefits realized by the systems of both the '089 Patent and the '652 Patent. Additionally, the source described in the '053 Patent can be made much more compact than the system disclosed in the '652 Patent and therefore can be more readily retro-fitted into older systems. However, the arrangement described in '053 Patent is unnecessarily complicated and practical models are difficult to design and manufacture due to the requirement to provide cooling and power utilities to three separate electrical components. A significant process concern for the design of the '053 Patent is that the main cathode is a current carrying electrode that is driven with a DC voltage. Since it is exposed to the process, it may be subject to coating with dielectric material. It is well known to those skilled in the art that this can lead to process instability through arcing.